Page 342 - A Practical Guide from Design Planning to Manufacturing

312 Chapter Ten

Straight pins used to be the most common type of lead. They are
required for THT mounting and give the package a solid physical as well
as electrical connection to the board. To allow packages to be mounted
on the surface of the board, the pin can be bent out horizontally once it
clears the bottom of the package. This is a gull-wing pin and is the sim-
plest pin to use with SMT. The flat portion of the pin gives a large area
to make a good contact to the board. The disadvantage of gull-wing pins
is that these outward pointing pins cause the package to take a large
amount of board area. Bending the pins inward instead of outward saves
area on the board. These are J-bend pins, named since their shape resem-
bles the letter “J.” A disadvantage of J-bend pins is that the soldered con-
nections to the board are now under the package, making them far more
difficult to inspect. Likewise soldering by hand or removing J-bend com-
ponents is far more difficult than a component with gull-wing pins.
The smallest and most direct connection possible is made using either
a ball lead or a land pad. Components using either of these types of con-
nections are sometimes called leadless packages because they lack pins,
although technically a lead is any electrical connector between the package
and the board. Ball lead packages have solder balls attached to package.
After the component is placed with these balls resting on metal pads on
the board, heat is applied to reflow the solder and make the connections.
Land leads are simple circular pads of metal. The board has solder
paste applied before the component is placed to form the needed solder
ball connections between the package land and the board. Although the
final connection is similar for ball lead and land lead packages, land lead
packages are less costly to produce but make assembly more difficult.
In theory, any of these types of leads could be used with a socket, but in
practice gull-wing and ball packages are rarely used that way. Sockets
must apply significant pressure in order to make a good electrical con-
nection without solder. Gull-wing pins bend easily and soft solder balls are
damaged easily. Straight pins are ideal for sockets. Aclamping mechanism
is used to grab the sides of the pin creating the electrical connection and
holding the package in place. Sockets for J-bend packages are made that
press in on the vertical sides of the pins. Placing pins on the board instead
of on the package creates a land lead socket. The pins point up nearly ver-
tical from the board and bend parallel to the board at their tips. The pack-
age rests on top of the pins with its pads touching the flat tips. Amechanical
clamp forces the package down onto the pins and holds it in place.
Sockets add cost but allow the end user of the board to replace bad
components or make upgrades easily. It is a huge advantage for a proces-
sor to be able to use the same socket as a previous generation. Sockets
also help with rapid prototyping. If many different chips are to be tested
with the same board, designing with a socket may be more of a con-
venience for the board manufacturer than the customer.